1. Field of the Invention
The present invention relates to a cooling tower distribution system which directs hot water to be cooled exclusively to water dispersing nozzles above outer regions of a fill assembly whenever the flow rate of hot water is less than a certain value, and which directs water to nozzles above the entire assembly, including the outer regions as well as a central region, whenever the flow rate exceeds the aforementioned certain value. Fixed structure is provided for equalizing the water pressure among all of the nozzles in current operation regardless of momentary or long term variations in the rate of water flow.
2. Description of the Prior Art
Water which has been used to condense steam generated by a boiler for production of electricity or in connection with other processes where steam is used as a motive fluid is often cooled at a point in the process cycle by means of a cooling tower. Electric utilities, for example, often employ very large, natural draft concrete towers wherein quantities of initially hot water are sprayed downwardly by nozzles toward a fill assembly, while natural, convective air currents rise through the tower including the fill assembly in direct opposition to the descending water for cooling the latter.
In general, towers are sized to provide sufficient cooling for the process water during summer days when the highest ambient air temperatures are approached and the volumetric flow rate of water through the tower is at a maximum, taking into account the overall performance of the tower. As a consequence, most of the time the tower produces cold water at a temperature below the design point. This is generally desirable as it increases the efficiency of the steam generating unit. There are times, however, when the power plant is operating at partial load and/or the ambient air temperature is below the freezing point of water. Under these conditions the general practice is to reduce the quantity of water being circulated to the tower to be cooled. The reduction in volumetric flow of water to the tower is usually accomplished either by bypassing the tower with a portion of the water used in the condensing process or by reducing the number of pumps used to supply water for the condensing process.
In these times of reduced water flow rates to the tower there are two primary concerns. For the tower to perform effectively, the water must be distributed uniformly. Also, when the ambient air temperature is below freezing, all portions of the tower which are directly exposed to the coldest air must have a concentration of water sufficient to minimize the formation of ice. The problem of ice is particularly troublesome because the air inlet face of the tower can become clogged to such an extent that air flow through the fill structure is significantly hindered, thereby lowering tower effectiveness. Moreover, damage to fill structure and other tower components can result due to the weight of the ice.
The nature of water distribution nozzles often used in cooling towers is another factor which must be taken into consideration during operation of the tower in relatively cold ambient conditions. One type of nozzle which has been found to be particularly useful for even, uniform distribution of water over fill structure at varying flow rates is the nozzle assembly described in U.S. Pat. No. 4,208,359 dated June 17, 1980 and assigned to the assignee of the present invention. In nozzles in general, including the type illustrated in U.S. Pat. No. 4,208,359, the flow rate of water through the nozzle is a function of the square root of the head of water encountered by the nozzle, and consequently a certain minimum, theoretically desired flow rate is established for proper operation of the nozzle so that the water is evenly distributed over the underlying fill structure in a uniform, wideangle dispersion pattern to avoid areas of low water concentration in the fill structure and formation of iced regions.
In some instances, the fill assembly of conventional counterflow towers is divided into a peripheral, outer region adjacent the tower shell defining the air passageway, and an inner region which is surrounded by the outer region. During winter operation of such towers, motor operated valves or gates are actuated to direct hot water to be cooled only toward the nozzles above the outer region of the fill assembly, so that as the volumetric flow rate of water is lowered in proportion to the reduced demands of the process, the nozzles distribute an adequate amount of water to the working regions of the fill structure to avoid ice formation. In natural draft towers, the supply of hot water to a peripheral region of the tower presents a water "curtain" to incoming air in order to partially counteract the natural convection of the tower that is enhanced during cold weather conditions.
Unfortunately, electrically actuated valves and gates for the relatively large diameter conduits carrying the hot water to the fill structure represent a significant cost, both in terms of initial capital outlay as well as expenses for periodic mechanical maintenance and electrical service. In addition, since the fill structure is generally open to the atmosphere, the cooling water often becomes contaminated with particulates such as twigs, leaves, scale and other objects which may tend to hinder operation of the valves. In some cases, plastic or synthetic rubber-like cleaning balls are intentionally placed in the cooling water to clear heat exchange tubes of the condenser, and such balls can occasionally become lodged in the valves, preventing operation and requiring manual intervention. Water for the process is sometimes drawn from a river with minimal treatment, and understandably foreign objects in river water can also be particularly troublesome.